Scanning tunneling microscopy (STM) has been used to demonstrate the nanocrystalline nature of the passive film on pure Fe formed at high anodic potential in near neutral borate buffer solution. The average lateral grain size was of the order of 5 nm. This confirms suggestions based on previous surface X-ray diffraction data. The oxide grains are clearly visible superimposed on the substrate metallic grains and form as a result of a rapid nucleation and growth process during electrochemical passivation. (C) 2002 The Electrochemical Society.